A review of higher order ionospheric refraction effects on dual frequency GPS
Petrie, EJ and Hernandez-Pajares, M and Spalla, P and Moore, P and King, MA, A review of higher order ionospheric refraction effects on dual frequency GPS, Surveys in Geophysics, 32, (3) pp. 197-253. ISSN 0169-3298 (2011) [Refereed Article]
Copyright 2010 Springer Science+Business Media B.V.
Higher order ionospheric effects are increasingly relevant as precision
requirements on GPS data and products increase. The refractive index of the ionosphere is
affected by its electron content and the magnetic field of the Earth, so the carrier phase of
the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system
design the polarisation is unaffected. Most of the effect is removed by expanding the
refractive index as a series and eliminating the first term with a linear combination of the
two signals. However, the higher order terms remain. Furthermore, transiting gradients in
refractive index at a non-perpendicular angle causes signal bending. In addition to the
initial geometric bending term, another term allows for the difference that the curvature
makes in electron content along each signal. Varying approximations have been made for
practical implementation, mainly to avoid the need for a vertical profile of electron density.
The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic
models such as the International Geomagnetic Reference Field. The largest effect is from
the second term in the expansion of the refractive index. Up to several cm on L2, it
particularly affects z-translation, and satellite orbits and clocks in a global network of GPS
stations. The third term is at the level of the errors in modelling the second order term,
while the bending terms appear to be absorbed by tropospheric parameters. Modelling
improvements are possible, and three frequency transmissions will allow new possibilities.
global positioning system, time series, higher-order effects, ionosphere, GPS, refractive index, signal bending